1. Field of the Invention
This invention relates generally to drill strings for drilling boreholes for the production of hydrocarbons and more particularly to a drilling assembly which utilizes a downhole controllable steering device for relatively accurate drilling of short-radius to medium-radius boreholes. The drilling assembly-of the present invention is particularly useful with coiled-tubing operations.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes or wellbores are drilled by rotating a drill bit attached to a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. More recently, demand for drilling short to medium radius wellbores has been increasing. The term “short radius wellbores” generally means wellbores with radii between 12 and 30 meters, while the term “medium radius wellbores” generally means wellbores with radii between 30 and 300 meters.
Modern directional drilling systems generally employ a drilling assembly that includes a drill bit at its bottom end, which is rotated by a drill motor (commonly referred to as the “mud motor”) in the drilling assembly. The drilling assembly is conveyed into the wellbore by a coiled tubing. A fluid (“mud”) under pressure is injected into the tubing which rotates the drilling motor and thus the drill bit. The state-of-the-art coiled-tubing drill conveyed drilling assemblies usually contain a drilling motor with a fixed bend and an orienting tool to rotate the high side of the drilling motor downhole in the correct direction. The-currently available coiled-tubing drilling assemblies (systems) with such orienting tools are typically more than sixteen (16) meters long. Tools of such length are difficult to handle and difficult to trip into and out of the wellbore. Furthermore, such tools require long risers at the surface. Such orienting tools require relatively high power to operate due to the high torque of the drilling motor and the friction relating to the orienting tool.
To drill a short radius or medium radius wellbore it is highly desirable to be able to drill such wellbores with relative precision along desired or predetermined wellbore paths (“wellbore profiles”), and to alter the drilling direction downhole without the need to retrieve the drilling assembly to the surface. Drilling assemblies for use with coiled tubing to drill short-radius wellbores in the manner described above need a dedicated steering device, preferably near the drill bit, for steering and controlling the drill bit while drilling the wellbore. The device needs to be operable during drilling of the wellbore to cause the drill bit to alter the drilling direction.
The present invention provides drilling assemblies that address the above-noted needs. In one embodiment, the drilling assembly includes a steering device in a bearing assembly which is immediately above the drill bit. The steering device may be operated to exert radial force in any one of several directions to articulate the drill bit along a desired drilling direction. The steering assembly may be disposed at other locations in the drilling assembly for drilling medium radius wellbores. Devices and/or sensors are provided in the drilling assembly to continuously determine the drilling assembly inclination, azimuth and direction. Other measurement-while-drilling (“MWD”) devices or sensors may be utilized in the drilling assembly, as is known in the drilling industry.